Relatively little is known about the molecular pathways that direct gonad formation in mammals. Defective gonadogenesis in humans can result in sterility or sex reversal, so it is important to understand the mechanisms that guide this developmental process. Sonic hedgehog (Shh) is a secreted signaling protein involved in the patterning and axis formation of many tissues in vertebrate and invertebrate species. Mice lacking a functional Shh gene product display a number of defects, including abnormal development of the limbs, axial skeleton and nervous system. A preliminary analysis has determined that gonadogenesis is also affected in Shh mutant mice. Gonads of Shh null embryos are reduced in size compared to controls and this phenotype is particularly severe in female embryos. In addition, the reproductive tracts of both males and females do not form properly. Three major specific aims are proposed to determine the roles of Shh in murine gonadogenesis. First, the expression pattern of Shh and its receptors in the developing gonad and the surrounding mesoderm will be determined. Second, the defects in gonad formation in the Shh -/- embryos and the temporal development of the mutant phenotype will be characterized. Finally, experiments will be performed to determine if Shh signals act directly on cells of the developing gonad or if Shh is involved in the development of the mesonephros, from which the gonad arises.